Chemical cellulose pulp is at present produced by delignifying wood chips with the aid of suitable cooking liquid in pulp digesters designed for this purpose.
Raw materials for pulp production are preferably various types of softwood, for example spruce or pine, or hardwood such as eucalyptus, birch or aspen. In recent times, alternative fibrous raw materials have also come into use to some extent in pulp production.
Examples of such raw materials are straw, reed-grass and esparto.
Today, most paper pulp is produced using continuous pulp digesters, of which there are at present a number of different types, for example hydraulic digesters and steam/liquor phase digesters.
The simplest type of steam/liquor phase digester is of the single-vessel type and is suitable for easily impregnatable wood types such as eucalyptus and other hardwood raw materials.
A further development of this type of digester is represented by the so-called two-vessel steam/liquor phase digesters which are provided with a separate pre-impregnation vessel in which the wood chips are saturated with the desired liquid before the actual delignification or cooking begins. Two-vessel steam/liquor phase digesters are especially suitable for wood types which are difficult to impregnate, for example pine.
Thus, there are a number of different types of digester arrangements which can be used for pulp digestion. In this connection, a number of different cooking liquids can be used for delignifying the wood and releasing the cellulose fibres. The active constituent in a cooking liquid often gives the name of the whole process, for which reason an alkaline cooking process with sulphur-containing cooking liquid is called a sulphate process, while an acid or in some cases neutral cooking process with sulphur-containing cooking liquid is called a sulphite process. Other known pulp digestion processes which may be mentioned, among others, are the soda process and the soda-anthraquinone process.
After the pulp digestion or delignification has taken place in a pulp digester, the pulp fibres released from the wood can be cleaned in different ways, for example by pulp washing, in which cooking liquid and released wood substances are removed, and by screening, in which impurities and non-defibred material such as shives and knots are removed.
In the case of unbleached pulp which is to be used in an integrated paper mill, the pulp is ready for use after the abovementioned cleaning stage and can be pumped onwards to a paper-making machine. In the case of a pulp mill for bleached market pulp, the pulp digestion and cleaning stage are followed by an additional bleaching sequence, a pulp wet-machining sequence, a drying sequence and usually also a baling sequence. In the case of flash-dried pulp, the pulp wet-machining sequence is omitted.
The reasons for pulp being bleached are on the one hand to remove remaining impurities and on the other hand to obtain a higher brightness of the pulp when this is required. The bleaching was previously carried out to a large extent using chlorine gas and sodium hydroxide in different sequences. Today, however, primarily for environmental reasons, a large number of new bleaching processes have been developed, in which chlorine gas has more or less been eliminated, and bleaching chemicals such as chlorine dioxide, hydrogen peroxide and ozone are increasingly used.
In the manufacture of chemical pulp, the pulp production process generally starts from pulp wood which is first debarked and cut up into chips in devices provided for this purpose, after which the chips are screened in order to remove impurities and chips of differing sizes. The chips are usually stored in the interim in a chip stack or in another similar chip store, and are thereafter transported to a chip silo, also called a chip bin, from which the wood chips are metered onwards in the process.
After the chip bin, or in some cases also inside the chip bin, the wood chips are usually treated with the purpose of removing air from the wood and preparing it for impregnation in a pre-impregnation stage. This is done by so-called steaming, which involves the wood chips being exposed to hot water vapour. In this steaming, the hot vapour drives out any air which is enclosed in the wood, at the same time as the wood chips are heated and saturated with moisture. This affords a certain softening of the wood material and prepares the chips for the impregnation phase, which is carried out in a subsequent process stage.
After the steaming stage, the wood chips are generally conveyed through at least one feeder. A feeder generally consists of a rotary vane feeder which allows wood chips to be passed through between vessels which are at different pressures. Depending on the pressure difference between the vessels which a feeder connects, such an arrangement is called either a low-pressure feeder or a high-pressure feeder. A high-pressure feeder allows the wood chips to be passed in to the pre-impregnation vessel, which is usually at a relatively high pressure. In the pre-impregnation vessel, the wood chips are impregnated with an impregnation liquid which generally consists of fresh and/or recycled cooking liquid and which in the case of sulphur-containing alkaline cooking liquids is called white liquor or black liquor.
After the pre-impregnation stage, the impregnated wood chips are transported onwards to a pulp digester for cooking.
In recent times, process developments within the area of pulp production have been increasingly aimed at reducing energy consumption and process discharges. Of course, the developments have also been aimed at reducing the production and investment costs and at simplifying manufacture, running and servicing.
Thus, international patent application number PCT/US/95/15458 describes a system and a method for feeding chips, which system and method are said to result in a simplified chip-feeding system for a continuous digester which can be used also for batch digesters.
The system is reported to comprise a single vessel with a top and a bottom, which vessel is said to have the same functions of a conventional chip bin, a conventional steaming vessel and a conventional chip trough, also called a chip chute.
The system described in PCT/US/95/15458 is said to afford an appreciable reduction in the costs involved in constructing and operating a chip-feeding system for a continuous digester.
It is further stated in PCT/US/95/15458 that the system described therein uses a process comprising: in a first stage, enclosing finely divided cellulose-containing fibrous material in a predetermined, open volume; thereafter, in the said volume, establishing a first level of fibrous material and a second level, beneath the first level, of cooking liquid; exposing the fibrous material between the first level and the second level to steam with the purpose of steaming the fibrous material; suspending the fibrous material with cooking liquid under the second level in order to produce a sludge; and removing the sludge from the said volume, pressurizing the sludge and feeding the pressurized sludge to a continuous digester.
In PCT/US/95/15458 it is also stated that the vapour added for steaming can, for example, originate from any available steam source in the mill, and can for example consist of fresh steam. However, it is stated that steam which has been produced from expanded cooking liquid may contain undesirable, totally reduced sulphur gases which have to be collected and destroyed, and that fresh steam is therefore preferred. It is further stated that the liquid used for impregnation is cooking liquid, for example black liquor, white liquor, green liquor or sulphite cooking liquid.
Something which may be experienced as a disadvantage of the previously disclosed technique is the difficulty in managing the foul-smelling gases which arise in the case where extraction liquor from a pulp digester is used for generating steaming vapour.
Another disadvantage which may be experienced with the previously disclosed technique is that the production of fresh steam for steaming, like the process as a whole, generally requires a great deal of energy.